docs: def add(self, data, key=None):
          """Add field values to container.
  
          :param dict data: Serialized values for field
          :param str key: Optional key; if not provided, values will be added
              to `self.payload`.
  
          """
          sink = self.options[key] if key is not None else self.data
          for key, value in iteritems(data):
              sink.add(key, value)